Copiers, facsimile machines and image scanners convert visual images into an electronic form suitable for printing, storage, transmission, or other computer and electronic uses. A system typically includes a light source, an array of photosensors, and electronics for converting analog photosensor outputs into digital data. Light is reflected off of an opaque image medium or light is transmitted through a transparent image medium and focused onto the photosensors. Color devices may have multiple light sources, each with a different band of wavelengths, or broad spectrum light may be split into multiple bands by a color separator, or filters may be employed.
For color imaging, a common configuration for the photosensors is three or more linear rows of CCD elements. In a typical arrangement, one row receives light in a band of red wavelengths, one row receives light in a band of blue wavelengths, and one row receives light in a band of green wavelengths. Typically, all of the CCD rows are exposed for a fixed period of time, after which the charges are transferred in parallel to analog shift registers. The charges are then serially shifted bucket brigade style to charge detectors. The charge detectors provide a voltage, the resulting voltage is amplified, and the resulting amplifier output voltages are converted into digital data streams by analog-to-digital converters (ADC).
Each of the elements in each light measurement channel (CCD, amplifier, ADC) has a finite range. Ideally, for maximum signal-to-noise, each element operates over its full range. That is, for the maximum expected light intensity at the CCD, the CCD almost saturates, the amplifier almost saturates, and the resulting ADC output is almost the maximum digital value. Similarly, ideally, for the minimum expected light intensity at the CCD, the CCD, the amplifier output and the ADC output are all their opposite operating extremes relative to the values at the maximum light condition. In general, light intensity and CCD sensitivity may vary from system to system and may vary within a system over time. In addition, the maximum and minimum light intensities at the CCD may be different for one color band than for another color band. In addition, the maximum and minimum light intensities at the CCD may depend on whether the light is reflected from an opaque image or transmitted through a transparent image. Therefore, there is a general need for adjustment of various parameters in each light measurement channel to maximize the signal-to-noise. Potential areas of adjustment include light intensity, CCD exposure time, and amplifier gain.
There have been numerous approaches to adjustment of the various parameters. In U.S. Pat. No. 4,408,231(Bushaw et al), a course exposure adjustment is provided by an ability to double the exposure time, and a fine adjustment is provided by adjusting the lamp intensity. In U.S. Pat. No. 4,839,739 (Tachiuchi et al), CCD exposure time is adjusted by varying the frequency of the shift clock. In U.S. Pat. No. 5,182,658 (Ishizaki), the exposure time is made approximately constant for a range of image contrast by terminating exposure early if certain conditions are met. In U.S. Pat. No. 5,479,207 (Degi et al and in European Patent Application EP 0 401 567 A2 (Nagano), the shift clock is a fixed frequency, the time required to shift and convert all the charges is less than the CCD exposure time, and the CCD exposure time is adjusted by varying the number of shift pulses after the last valid charge has been converted.
There is a need for further improvement in adjustment for maximum signal-to-noise. In particular, there is a need for independent adjustment for each color in a color imaging device to maximize the signal-to-noise for each color.